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Nutation in Ulysses’ Third Orbit: Nutation in Ulysses’ Third Orbit Daniel Presti, Colin Goulding
Presentation Overview: Presentation Overview The Ulysses mission Ulysses’ nutation anomaly Nutation in the third orbit Analysis of the past nutation season and predictions for the upcoming season Operations concepts
Mission Overview: Mission Overview 6 October 1990: Launch (Space Shuttle Discovery) 1992: Jupiter flyby and injection into polar orbit 1994-1995: First set of polar passes 2000-2001: Second set of polar passes 2006-2008: Third set of polar passes
The Ulysses Spacecraft: The Ulysses Spacecraft Spin-stabilised at 5 rpm Thermally isolated Two internal platforms Power provided by RTG Two 35 m wire booms One axial boom One radial boom Attitude control provided by hydrazine thrusters High-gain antenna (1.65 m) for X-Band downlink and S-Band uplink
Instruments and Investigations: Instruments and Investigations
The Nutation Anomaly: The Nutation Anomaly Caused by thermal distortions of the axial boom, when partially illuminated by the Sun Occurs around perihelion Spacecraft “wobbles” about its spin axis, coupling energy into wire booms Can grow exponentially, if left uncontrolled
Nutation Forcing Function (NFF): Nutation Forcing Function (NFF) Quantifies the intensity of nutation (build-up rates) Based only on solar aspect angle (SAA) and range (R) SAA determines the fraction of the boom illuminated by the Sun R determines the intensity of the incident radiation
Nutation Control: Closed Loop CONSCAN: Nutation Control: Closed Loop CONSCAN Onboard attitude control system Requires a stable uplink from a ground station for closed-loop control Estimates the Earth off-pointing (amplitude and phase) by measuring physical properties of the receiver’s AGC signal (conical scanning) Fires thrusters to re-align the spin axis with the Earth Reduces the Earth off-pointing Damps the nutation (secondary effect) Two pre-defined selectable deadbands (wide and narrow)
The Third Orbit’s Nutation Season: The Third Orbit’s Nutation Season From February 2007 to February 2008 NFF comparable to second orbit’s season Lower RTG power Minimal power margin Mutually-exclusive operations Reduced DSN ground station coverage Large gaps with uncontrolled nutation build-up
Analysis of Previous Nutation Season: Analysis of Previous Nutation Season Based on high-rate attitude data from the 2001 season (instantaneous Earth off-pointing and nutation angle) Empirical characterisation of the nutation build-up rates and CONSCAN’s nutation damping performance
MAGIC: MAGIC Maximum Allowable Gaps In CONSCAN operations have been defined, based on predicted nutation build-up rates As gaps in nutation control increase: Nutation increases above acceptable levels (science-defined) Loss of downlink due to off-pointing Severe increase in unwanted oscillations in wire booms Loss of spacecraft?
Ground Station Coverage Requirements: Ground Station Coverage Requirements Maximum acceptable nutation limit defined as 0.5° half-cone (science-driven) Resulting gaps in station coverage – based on maximum gaps in CONSCAN, minus an operational overhead Almost-continuous coverage required at the peak of the nutation season
Typical Nutation Operations Sequence: Typical Nutation Operations Sequence Two nutation-control CONSCAN session per pass Uplink of commands during CONSCAN sessions High-rate playback of recorded data in between CONSCAN sessions Gap in coverage (< MAGIC) with nutation build-up and data recording
DSN Visibility: DSN Visibility DSN designed primarily for in-ecliptic spacecraft Non-continuous visibility between March and July 2007 DSN gap exceeds maximum allowable gap!
Use of Kourou: Use of Kourou The ESA Kourou station can fill the gap almost entirely Negotiations for the use of other stations for nutation control are currently taking place
Conclusions: Conclusions The Ulysses extended mission operations are challenging due to the diminishing resources The nutation anomaly increases the challenge significantly Nutation control has to take place with reduced coverage and mutually-exclusive spacecraft operations A detailed characterisation of the past nutation season took place, leading to: An improved understanding and quantification of the nutation phenomenon A proven need for additional ground resources during critical phases

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